1. Field of the Invention
The present invention relates to a technology for controlling speed of an endless belt by a feedback control performed based on detection of scale marks formed on the belt.
2. Description of the Related Art
An image forming apparatus generally includes a belt-speed control device that controls the speed of an intermediate transfer belt. A plastic scale seal with scale marks is adhered to the periphery of the intermediate transfer belt. A sensor (reflective photosensor) reads the scale marks and outputs detection pulses. Based on the detection pulses, the speed of the intermediate transfer belt is controlled by controlling a belt driving motor that drives the intermediate transfer belt. Thus, the speed of the intermediate transfer belt can be stabilized at an ideal speed.
As a result, variations in the speed of a sheet carrying belt and the intermediate transfer belt can be prevented even in a tandem-type color image forming apparatus including a plurality of photosensitive members 91Y, 91C, 91M, AND 91K, as shown in FIG. 9 and FIG. 10. Consequently, misalignment of toner images can be reliably prevented.
However, there is generally a seam, or a gap, between the front and rear ends of the intermediate transfer belt. Upon encountering the gap, the sensor outputs detection pulses with a wider interval (see FIG. 11) than otherwise (see FIG. 12).
Because of the wider pulses, a control system misjudges that the belt has slowed down, and erroneously performs feedback control to increase the belt speed.
One approach is to control the belt speed by a dummy pulse stored in a RAM, etc., instead of using the pulse output by the sensor when the gap passes under the sensor.
The average interval of the dummy pulses is the same as that of the pulses output by the sensor when reading the scale marks while the intermediate transfer belt is being driven at an ideal speed. Thus, by using this dummy pulse, the belt can be driven at an ideal speed even when the gap is encountering the sensor.
When it is determined from the detection pulses that the gap is encountering the sensor, the control system controls the belt speed by using the dummy pulse. Thus, there is a need to quickly determine that the gap is encountering the sensor.
If there is a delay in determining that the gap is encountering the sensor, the control system might continue using the detection pulse output from the sensor during the delay period and perform feedback control of the belt speed based on the detection pulses. In this case, the belt speed is erroneously increased before employing the dummy pulse. As a result, the belt speed cannot be accurately controlled.
The sensor generally cannot immediately detect the gap when the front end of the scale reaches the sensor. The reason for this is because of the characteristic of an analog voltage output that is output by the sensor upon reading the scale marks of the scale. FIG. 13 depicts the characteristic of the analog voltage output that is output by the sensor upon reading the scale marks of the scale.
An output voltage value α represents the output voltage value when the sensor is reading the scale marks. As the gap reaches the sensor, the voltage gradually drops. When the output voltage drops to a threshold β or less, the control system recognizes that the gap region has begun from this point on, and switches to controlling the belt speed based on the dummy pulses instead of the detection pulses.
Accordingly, at the portion marked “x” in FIG. 13, the control system does not implement control using the dummy pulse even though the actual gap region has crossed the sensor. Consequently, the belt speed is controlled inaccurately, resulting in misaligned toner images and leading to degradation of color image.
To solve this problem, a belt-speed control device having two sensors has been proposed. When a first sensor, provided upstream in the direction in which the belt is driven, detects the gap, a second sensor, provided downstream in the direction in which the belt is driven, controls the belt speed.
However, the second sensor takes over the control only after the first sensor recognizes the gap, resulting in the delay as denoted by “x” in FIG. 13. Therefore, the problem remains unsolved.
Japanese Patent Laid-Open Publication No. 2004-69933 discloses two examples of another belt-speed control device. In the first example, the surface of an endless belt is covered with a linear scale (scale seal) having a plurality of timing scale marks (pitches) along the circumferential direction. Three sensors with spaces therebetween in the circumferential direction are provided along the linear scale. Two of the sensors simultaneously read the linear scale and each sensor outputs a signal. A linear encoder receives the signals from both the sensors and synchronizes the pulse timings of the pulse signals of the two sensors. A controller controls the belt speed based on the signal output from the linear encoder.
In a second example of this conventional belt speed detecting device, two linear scales forming two columns in the breadth direction of the endless belt are provided at shifted positions in the circumferential direction of the belt in such a manner that their edges overlap with each other. Two sensors are arranged so that each sensor reads one of the linear scales.
However, in the first example, controlling the speed requires complex software to synchronize the timings of the pulse signals output by the two sensors.
In the second example, the belt must be wide enough to accommodate two linear scales, which makes the scale of the device bigger. Further, arranging the two linear scales perfectly parallel to each other is a difficult task.